The present invention relates generally to a signal transmission system implemented with optical fibers and related optical components. More particularly, this invention relates to an optical interleave device implemented in a dense wavelength division multiplexing (DWDM) system.
As the optical wavelength division multiplexing (WDM) technology gradually becomes the standard backbone network for the fiber optic communication systems, a challenge is continuously faced by those of ordinary skill in the art to increase the transmission capacity due to the bandwidth limitations of the optical fiber signal transmission systems. Specifically, the bandwidth of the optical fiber amplifier, such as Erbium doped fiber amplifier (EDFA), is limited as more and more channels are inserted into the transmission band. The wavelength spacing between adjacent channels employed for carrying the optical signals becomes narrower when more channels are xe2x80x9csqueezed inxe2x80x9d the transmission band for the purpose of satisfying a requirement of increasing the transmitting capacity of the signal transmission system. However, the wavelength division multiplexing (WDM) technologies employing dielectric filters are confronted with a limitation due to the ability to separate one channel from adjacent ones when the channel spacing is further reduced with increased number of channels.
As the WDM technology now enables the utilization of substantially wider fiber bandwidth for signal transmission, a number of prior art patents disclosed methods and configurations deal with problems of multiplexing, demultiplexing, and routing optical signals such that these systems can become commercially viable. Particularly, these disclosures deal with problems arise from addition of the wavelength domain that increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of coupling many wavelengths in the same fiber. Demultiplexing is the opposite process in which wavelengths coupled in the same fiber are separated. The individual channels are spatially separated and coupled to specific output ports, Routing differs from demultiplexing in that a router spatially separates the input optical channels into output ports and permutes these channels according to control signals to a desired coupling between an input channel and an output port.
Lyot Orman invented a first birefringent filter in 1933, and in 1953, Solc further disclosed a different type of birefringent filter. These two types of birefringent filters provide a special function of selecting desired wavelengths in certain polarization state and in the meantime, select the adjacent wavelength with a perpendicular polarization state. Lyot""s birefringent filter is previously employed by a prior art Patent for a switchable wavelength router. Specifically, a conventional switchable router is disclosed by Wu et al. in U.S. Pat. No. 5,694,233 entitled xe2x80x9cSwitchable Wavelength Routerxe2x80x9d (issued on Dec. 2, 1997) and is now incorporated herein by reference for all purposes in the Patent Application. Wu et al. disclosed a switchable wavelength router as that shown in FIG. 1. The switchable wavelength router 999 has a first birefringent element 30 that decomposes and spatially separates an incoming WDM optical signal into two orthogonally polarized beams 101 and 102. A first polarization rotator 40 selectably rotates the polarization of one of the beams to match the polarization of other beam, based on an external control signal. A wavelength filter 61 (e.g., stacked waveplates) provides a polarization-dependent optical transmission function such that the first beam decomposes into third and fourth orthogonal beams, and the second beam decomposes into fifth and sixth orthogonal beams. The third and fifth beams carry a first spectral band at a first polarization and the fourth and sixth beams carry a second spectral band at an orthogonal polarization. A second birefringent element 50 spatially separates these four beams into four horizontally polarized and vertically polarized components. A second polarization rotator 41 rotates the polarizations of the beams so that the third and fifth beams and the fourth and sixth beams are orthogonally polarized. A third birefringent element 70 recombines the third and fifth beams (i.e., the first spectral band), and also recombines the fourth and sixth beams (i.e., the second spectral band) which are coupled to the two output ports based on the control state of the wavelength router. Wu et al. disclosed a switchable wavelength router having an input port for the incoming WDM signals and a two output ports. The router divides the received optical signals into divided optical signals comprising a subset of the channels and spatially positions the divided optical signals in response to a control signal applied to the router. Wu""s router can divide a received WDM signals into two subsets that are either single channel or WDM signals.
The disclosure of Wu et al. as discussed above is useful for switchable wavelength routing. It is also useful for a wavelength selector if there is no controlled polarization rotator. However, the router as disclosed is not practical for component miniaturization. Particularly, the arrangement of the two separate output ports 14 and 15 requires two independent collimators separated in certain distance due to the physical shapes and sizes of these collimators. Consequently, as the beams are separated with a spatial distance, a large crystal 50 is required and that would increase the production cost of the router and make the device quite expensive. Long optical path according to Wu""s configuration would also increase the insertion loss and increase the volume, size and weight of the router assembly.
In view of the existing technologies, besides the challenge of processing transmission signals carried by wavelengths narrower channel spacing, another challenge in constructing an optical transmission system is to provide miniaturized optical components with improved performance, compact size and lower cost. The task of miniaturization is often difficult to achieve. This is due to the particular reasons that the spacing between adjacent channels in the WDM system is around 0.8 nm and even lower and the device is very sensitive to the accuracy of the central wavelength of the pass band and pass band profile. Additionally, there are stringent requirements for other optical performance parameters such as polarization mode dispersion, polarization dependent loss and other thermal-dependent effects imposed on different operational parameters. Weight and size reductions to miniaturize an optical module such as an interleaver cannot be conveniently achieved taking into consideration of these complexities and design constraints.
Therefore, a need still exists in the art of manufacturing and designing the fiber optic interleaver to provide improved configurations that would reduce the required optical components. Specifically, novel and improved interleaver configurations utilizing less components and can reduce manufacture process to lower the manufacture cost are needed to resolve the difficulties and limitations encountered by the fiber optical industries such that compact fiber optical interleaver can be manufactured at a low production cost.
It is therefore an object of the present invention to provide an improved design and configuration for manufacturing and assembling a fiber optic interleaver with reduced number of components to reduce the weight and volume and lower the production costs. With the improved design and configuration, the aforementioned difficulties and limitations in the prior art can be overcome.
Specifically, it is an object of the present invention to provide an interleaver implemented with improved configuration by employing an incline beam angle directing means for projecting the output beams with small incline angles. Preferably, the incline angle directing means may be a prism for projecting the output beams with small incline angles such that these output beams are received directly into a dual fiber collimator. The optical components are reduced and the optical paths are shortened. Insertion loss is reduced and the production cost is lowered with the improved configuration disclosed in this invention.
Briefly, in a preferred embodiment, the present invention includes an optical interleaver. The optical interleaver includes an input port for receiving an input light beam including a plurality of wavelengths represented by xcex1, xcex2, xcex3, . . . , xcexn where n is a positive integer. The interleaver further includes a group of optical elements for decomposing the input light beam into a first light beam including a first set of wavelengths represented by xcex1, xcex3, xcex5, . . . , xcexn in certain state of polarization (SOP) and a second light beam including of a second set of A wavelengths represented by xcex2, xcex4, xcex6, . . . , xcexnxe2x88x921 in perpendicular state of polarization (SOP), wherein the first light beam and the second light beam transmitted respectively in a first and second optical paths, and the group of optical elements further decomposing and generating two sets of mutually orthogonally polarized and parallel beam-components from the first beam and second beam. The interleaver further includes an incline angle means coupled to the group of optical elements. The incline angle means is used for projecting the two sets of mutually orthogonally polarized and parallel beam-components for the first and second beam respectively with a first incline angle and second incline angle relative to an optical axis of the interleaver. And, the interleaver further includes an output beam-component combining means for combining the mutual orthogonally polarized and parallel beam-components into a first output beam and a second output beam projected with the first and second incline angles respectively relative to the optical axis of the interleaver. In a preferred embodiment, the interleaver further includes a dual fiber collimator having a first optical fiber and a second optical fiber disposed off-axis of the collimator for directly receiving the first output beam and the second output beam projected respectively with the first and second inclined angles.
In summary, this invention discloses an interleaver for separating a multiple wavelengths of a light beam into a first output beam and a second output beam having two different sets of wavelengths. The interleaver includes an incline angle means for projecting the first output beam and the second output beam with a small incline angle relative to an optical axis of the interleaver for the purpose of directly projecting into a first and a second optical fibers of an output dual-fiber collimator.
This invention further discloses a method of applying an interleaver for separating a multiple wavelengths of an input light beam into a first output beam and a second output beam having two different sets of wavelengths. The method includes a step of employing an incline angle means for projecting the first output beam and the second output beam respectively with a first and a second small incline angles relative to an optical axis of the interleaver. In a preferred embodiment, the method further includes a step of employing an output dual-fiber collimator provided with a first and second optical fibers for directly coupling to and receiving from the interleaver the first beam and the second beam respectively each projected with the small incline angle.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures.